1. Field of the Invention
The present invention relates to aqueous copolymer dispersions with low solvent content and based on hydroxy-functional copolymers, to a process for preparing them, to binder compositions containing the aqueous copolymer dispersions, and to the use of the binders for preparing scratch-resistant coatings.
2. Description of Related Art
In coating systems (for example, as described in DE-A 3 209 421, EP-A 95 263, EP-A 105 293, EP-A 133 949, EP-A 288 763) it is known to use water-dilutable, copolymer-based binders. These binders, however, generally include emulsifiers for stabilizing and/or sizable fractions of organic cosolvents.
The use of sizable amounts of organic solvents is undesirable on environmental grounds. It is, however, unavoidable in pursuit of the aim, during polymer preparation, of ensuring adequate stirrability and heat removal of the reaction mixture and a certain minimum fill level of the reactor. In aqueous coating compositions, additionally, organic solvents lead to advantageous effects such as improved storage stability, pigment wetting film optical qualities, and levelling.
Subsequent reduction of process-related solvent content from copolymers or copolymer dispersions entails a high level of expenditure in terms of apparatus and energy and, thus, also in terms of costs, so that there is a need for aqueous polymer dispersions that can be prepared largely without the use of organic solvents without any deterioration in performance properties.
Copolymer dispersions which are cured by means of a chemical reaction, for example with an amino resin or with a blocked or non-blocked polyisocyanate, must contain a certain amount of reactive groups, such as hydroxyl groups. These groups are introduced into the copolymer generally through the use of hydroxy-functional (meth)acrylic esters during the copolymerization. In comparison to the non-functional (meth)acrylic esters or to styrene, however, these hydroxy-functional raw materials are very expensive. Also, it is frequently necessary to use sizable amounts of these raw materials in comparison to copolymers in organic solution, in order to compensate the hydrophilicity of the coating films by means of a greater crosslinking density.
One method for the preparation of hydroxy-functional secondary copolymer dispersions that largely avoids the use of solvents for the polymerization is disclosed in EP-A 0 758 007. The solvents typically employed are fully or partly replaced therein by hydroxy-functional polyethers. The hydroxy-functional polyethers remain, as reactive diluents, in the secondary dispersion and are included in the subsequent crosslinking reaction with blocked or non-blocked isocyanates, forming urethane. Consequently they do not contribute to the VOC. It has become known, however, that in certain fields of application polyether fractions in the coating compositions lead to low resistance properties, for example to a low scratch resistance.
It has now been found that aqueous copolymer dispersions with a low solvent content and a high resistance level on the part of the coating films, particularly very good scratch resistance and acid resistance, can be based on hydroxy-functional copolymers if hydroxy-functional polycarbonates are used as reactive diluents.